Head-mounted display (HMD) devices are configured to be worn on, or otherwise affixed to, a user's head. An HMD device may comprise one or more displays positioned in front of one, or both, of the user's eyes. The HMD may display images (e.g., still images, sequences of images, and/or videos) from an image source overlaid with information and/or images from the user's surrounding environment (e.g., as captured by a camera), for example, to immerse the user in a virtual world. HMD devices have applications in medical, military, gaming, aviation, engineering, and various other professional and/or entertainment industries.
Many HMD devices use liquid-crystal display (LCD) technologies in their displays. An LCD display panel may be formed from an array of pixel elements (e.g., liquid crystal cells) arranged in rows and columns. Each row of pixel elements is coupled to a respective gate line, and each column of pixel elements is coupled to a respective data (or source) line. A pixel element may be accessed (e.g., updated with new pixel data) by driving a relatively high voltage on a gate line to “select” or activate a corresponding row of pixel elements, and driving another voltage on a corresponding data line to apply the update to the selected pixel element. The voltage level of the data line may depend on the desired color and/or intensity of the target pixel value. Thus, LCD display panels may be updated by successively “scanning” the rows of pixel elements (e.g., one row at a time), until each row of the pixel array has been updated.
The voltage applied on the data line changes the color and/or brightness of the pixel element by changing the physical state of (e.g., rotating) the particular pixel element. Thus, each pixel element may require time to settle into the new state or position. The settling time of a particular pixel element may depend on the degree of change in color and/or brightness. For example, transitioning from a maximum brightness setting (e.g., a “white” pixel) to a minimum brightness setting (e.g., a “black” pixel) may require greater settling time than transitioning from an intermediate brightness setting to another intermediate brightness setting (e.g., from one shade of “gray” to a different shade of “gray”). The delay in pixel transition may cause ghosting and/or other visual artifacts to appear on the display when the settling time of the pixel elements is slower than the time between successive frame updates.
LCD overdrive is a technique for accelerating pixel transitions when updating an LCD display. Specifically, a pixel element is driven to a higher voltage than the target voltage associated with the desired color and/or brightness level. The higher voltage causes the liquid crystal to rotate faster, and thus reach the target brightness in less time. On fixed LCD displays (e.g., televisions, monitors, mobile phones, etc.), an object is often illuminated by the same pixel elements for the duration of multiple frames. Thus, the amount of overdrive applied to the pixel elements of a fixed LCD display can be approximate since the user may be unable to detect errors in the corresponding pixel color and/or brightness when such errors last only a single frame. However, on HMD devices, and particularly in virtual reality (VR) applications, an object viewed on the display may be illuminated by different pixels as the user's head and/or eyes move. Therefore, the amount of overdrive applied to each pixel element of an HMD display should be much more precise to preserve the user's sense of immersion in the virtual environment.